Generally, exhaust gases issuing from various types of combustion apparatus contain NO.sub.x, occasionally in conjunction with other gaseous components such as sulfur oxides (hereinafter referred to as SO.sub.x). The NO.sub.x constitutes the principal cause of the so-called photochemical smog which has posed as a serious social problem. From the standpoint of prevention of air pollution, it is necessary that such an exhaust gas should be released into the atmosphere only after it has been freed from NO.sub.x.
For the removal of NO.sub.x from NO.sub.x -containing exhaust gases, there have heretofore been proposed numerous methods including so-called wet absorption methods and so-called dry vapor-phase catalytic reduction methods. The dry vapor-phase catalytic reduction methods are those which use hydrocarbons like methane, carbon monoxide, hydrogen and ammonia, for example, as the reducing agent for NO.sub.x. Of these catalytic reduction methods, that which involves the use of ammonia is extensively employed for the removal of NO.sub.x from industrial exhaust gases containing a major proportion of oxygen in conjunction with a minor portion of NO.sub.x. This is because the reaction of ammonia with NO.sub.x proceeds practically selectively even in the presence of oxygen.
The method, which makes use of ammonia, includes contacting the NO.sub.x -containing exhaust gas with ammonia in the presence of a catalyst generally at temperatures in the range of from 200.degree. C to 400.degree. C so as to react the NO.sub.x with ammonia, thereby converting the NO.sub.x into N.sub.2. In this method, the oxide of a metal such as Mn, Cu, Cr, V, Fe, Ti or Sn is used as the catalyst. Where there is used the oxide of such a metal, however, if the exhaust gas happens to contain SO.sub.x, then the metal oxide catalyst is poisoned by the SO.sub.x and consequently is prevented from advantageously manifesting its catalytic activity, with the result that the efficiency of removal of NO.sub.x is reduced. This method has also an economic disadvantage that the oxide of such a metal is generally rather expensive. Further, in the use of the metal oxide as the catalyst, it is frequently observed that traces of foreign matter contained in the oxide will bring about the so-called poisoning effect of heavily retarding or completely inhibiting the catalytic reaction. To prevent this poisoning effect, it is necessary to use a metal oxide of high purity obtained by the so-called impregnation method or precipitation method. A metal oxide of such high purity, however, is not easy to produce. In spite of such a high purity, a metal oxide is not completely free from the poisoning effect of SO.sub.x.